AlGaN/GaN based high electron mobility transistors (HEMTs) have been intensively used due to their high efficiency power switching and large current handling capabilities. However, the high power densities and localized heating in these devices form small, high temperature regions called hotspots. Analysis of heat removal from hotspots and temperature control of the entire device is necessary for the reliable design of HEMT devices. For accurate analysis of heat. transfer using thermal simulations in such devices with heat transfer occurring at different length scales, a roadmap is needed. For this purpose relative importance of different heat transfer modes in removing heat from devices with different substrate materials, operating at different power densities while different boundary conditions are analyzed using two and three-dimensional COMSOL Multiphysics simulations. Results give the relative importance of different parameters on the heat removal mechanism from devices and provide a roadmap for building simpler yet still accurate thermal models for AlGaN/GaN HEMTs and similar devices.